Night vision systems allow a vehicle occupant to better see objects during relatively low visible light level conditions, such as at nighttime. Night vision systems typically are classified as either passive night vision systems or active night vision systems. Passive systems simply detect ambient infrared light emitted from the objects within a particular environment. Active systems utilize a near infrared (NIR) light source to illuminate a target area and subsequently detect the NIR light reflected off objects within that area.
Active systems provide improved resolution and image clarity over passive systems. Active systems utilize laser or incandescent light sources to generate an illumination beam in the near infrared spectral region and charge-coupled devices or CMOS cameras to detect the reflected NIR light.
In an active night vision system a NIR laser is used to illuminate a target area. A camera is used in conjunction with the laser to receive the reflected NIR light from objects within the target area. The laser may be pulsed with a duty cycle and the camera may be operated in synchronization with the laser to capture an image.
The camera typically contains a band-pass filter that allows passage of light that is within a narrow range or band, which includes the wavelength of the light generated by the laser. The combination of the duty cycle and the use of the band-pass filter effectively eliminates the blinding effects associated with headlamps of oncoming vehicles. The term “blinding effects” refers to when pixel intensities are high due to the brightness of the oncoming lights, which causes an image to be “flooded out” or have large bright spots such that the image is unclear. However, the use of a laser and a corresponding filter can add costs and complexity to a night vision system. In addition to the laser and filter component costs and added circuit complexity, there are costs and added complexity associated with the thermal control of the laser. The wavelength of a laser is temperature dependent. In order to maintain a laser within a desired operating band, the temperature of the laser must be maintained. Moreover, a laser based night vision system also uses a fan optic, to shape and create an effective laser dispersion pattern, which can further add costs.
Also, although laser-based night vision systems can overcome the blinding effects of oncoming headlamps, they cannot overcome the blinding effects associated with highly reflective objects. For example, many signs have highly reflective surfaces for reflection of incandescent light, such as that emitted from vehicle headlamps, for direct viewing ease by a vehicle operator. The signs are often covered with retroreflective paint that can reflect a large amount of light and cause image saturation. A saturated image is generally unclear and unreadable. Large flat surfaces, such as on trucks, buses, and vans, can also cause image saturation.
In addition to the blinding effects of oncoming headlamps on a night vision system, oncoming headlamps can also degrade human vision of a vehicle operator. Illumination from oncoming headlamps that is directed at a host vehicle can obstruct the vision of a vehicle operator limiting the view of the roadway ahead of the host vehicle.
Although the headlamps of a vehicle when operated in “high-beam” mode in general provide a clearer view of the roadway ahead, high-beam mode operation can further obstruct the vision of an oncoming vehicle. As a result, high-beam mode operation is limited to instances when oncoming vehicles are not present. By only operating the headlamps when oncoming vehicles are not present, the host vehicle operator vision is also limited during oncoming vehicle instances when high-beam operation may be desired.
Thus, there exists a need for an improved active night vision system and method that prevents blinding of night vision system components and limits the directing of light at an oncoming vehicle. It is also desirable that the active night vision system and method provide reduced system complexity and costs and provide increased safety associated with vehicle operation.